1. Field of the Invention
The invention relates to a device for detecting a magnetic field, comprising a Wheatstone bridge with at least four magnetoresistive elements on a substrate, each magnetoresistive element comprising at least one sensitive portion comprising successively a first ferromagnetic layer having a magnetic easy axis extending in a first direction, a non-magnetic layer and a second ferromagnetic layer having a magnetic easy axis extending in a second direction that is different from the first direction, said sensitive portions having mutually parallel sensitive directions that are parallel to a third direction, each magnetoresistive element being associated with a current conductor provided in the immediate vicinity of that magnetoresistive element.
2. Description of Related Art
A device of this kind is known from WO-A-95/28649 (PHN 14.825). In the known device the first ferromagnetic layer exhibits uni-axial anisotropy in one direction due to its growth in a magnetic field. The magnetization of the second ferromagnetic layer is determined by exchange interaction with an antiferromagnetic layer provided on the second ferromagnetic layer. This interaction effectively results in uni-directional anisotropy of the second ferromagnetic layer. During manufacturing of the known device the second ferromagnetic layer and the antiferromagnetic layer are exposed, at a temperature higher than the blocking temperature, to a magnetic field which is generated by an electric current flowing in the current conductors, after which the temperature of the layers is lowered to a value below the blocking temperature whereas the magnetic field is sustained. This step fixes the direction of the effective anisotropy of the second ferromagnetic layer during the production, and hence the sensitivity of each magneto-resistive element in the Wheatstone bridge. In fact the Wheatstone bridge is programmed by this step to be sensitive for fields in a predetermined direction. The blocking temperature is the temperature at which the exchange bias field of the second ferromagnetic layer is substantially zero. The blocking temperature of, for example a suitable FeMn alloy is 140.degree. C. Consequently, the programming of the Wheatstone bridge requires a heat treatment and the device is thermally stable only sufficiently well below the blocking temperature.
For measurements of small static magnetic fields, a trimmer resistor has to be connected in series with at least one of the magnetoresistive elements of the known device. The value of this trimmer resistor is adjusted during manufacture of the device in order to compensate for unbalance of the bridge caused by deviations of the elements in the bridge relative to one another. This is a time-consuming and expensive operation.